WG C4.410 Lightning Striking Characteristics to Very High Structures

Background:

Lightning protection is one of the most important problems for insulation design of power transmission systems. For evaluation of lightning performance of transmission lines, the electro-geometrical model (EGM-model) has been used. However, it was reported that the observed shielding failure rate of UHV transmission lines, whose average height is about 100 meters, was larger than the value estimated with the conventional EGM model. Through detailed observation of lightning hits on these UHV lines, difference between the actual phenomena on such tall transmission lines and the conventional EGM has gradually been revealed.

Recently very high buildings and towers, whose heights are about 500 meters or more, have been constructed or planned all over the world. InJapan, a new TV tower named Tokyo Sky Tree, whose height is 610 m, will be constructed inTokyoby 2012. For such a very high structure, lightning protection design is important not only to prevent physical damage of the structure itself but also not to cause breakdown or malfunction of the low-voltage system and electronic equipments inside the structure. IEC TC 81 published IEC 62305 on the general principles of protection against lightning threatening structures and electrical and electronics systems within structures. In the document, the rolling sphere method is adopted to evaluate the performance of external lightning protection systems of a structure. Nevertheless, applicability of the rolling sphere method to very high structures has not been verified.

Though lots of observations of lightning have been reported so far, striking characteristics of lightning to such a very high structure have not been fully investigated yet. Several novel models to evaluate upward leader initiation from earthed objects or lightning interception ability of external lightning protection systems have been proposed, however, comparison with recent observational data on lightning striking characteristics is necessary to show the validity of the proposed models.

Discharge phenomena have basically stochastic characteristics and it can be said that lightning, as a natural phenomenon, does not behave in the same manner as before. Large amount of dataare, therefore, needed to understand the characteristics. In order to establish better lightning protection measures for tall transmission lines and very high buildings and towers, it is necessary to collect and analyse observation data of lightning striking such high structures.

Scope:

The scope of the working group is to:

Summarize the observed data related to the striking characteristics of lightning strokes to tall transmission lines and very high isolated structures.

Investigate the relationship between lightning striking characteristics to high structures such as annual or seasonal number of lightning strikes on them, proportion of upward lightning, height of the earthed objects and lightning flash density of the area.

Investigate the shielding characteristics of earth wires on tall transmission lines and of external lightning protection systems installed on very high isolated structures.